STRUCTURE AND DEVELOPMENT OF INSECTS 



27 



tissues. Constantly flowing around tne respiratory tubes or 

 tracheae, the blood is quickly and thoroughly purified, though the 



FIG. 20. Mouth-parts of a plant- 

 louse: a, the jointed beak; b, the 

 lancets, much enlarged; c, antenna; 

 d, foot. (After J. B. Smith.) 



Fio. 21. Diagram of tracheal 

 or breathing system of an 

 insect: sp., spiracles; tr., 

 trachea. (After Kolbe.) 



exact manner in which this is done is not definitely known. The 

 respiratory system has absolutely no connection with the mouth 

 or pharynx (Fig. 23, ph), as have the lungs of the higher animals, 



Fio. 22. Ideal section through an insect: a, alimentary canal; h, heart; 

 n, nerve cord; s, spiracle; t t tracheal tubes; I, legs; w, wings. (From 

 Riverside Natural History.) 



